The present invention relates generally to methods and apparatuses for detecting defects in optical fibers, and more particularly, to methods and apparatuses for detecting airlines in optical fibers.
The ability to detect defects in optical lightguide fibers is critical in providing high quality fiber and in devising manufacturing techniques that minimize the occurrence of such defects. Defects or inhomogenieties can affect the strength or transmission characteristics of the optical fiber. One class of defects, loosely defined as xe2x80x9cbubblesxe2x80x9d or xe2x80x9cairlinesxe2x80x9d, can range from the sub-micron (xcexcm) to several microns in diameter and multiple-meter lengths. Although the term xe2x80x9cairlinexe2x80x9d is used, defects can take on many different shapes and geometries. Defects over 4 xcexcm in diameter can cause a variety of problems, including proof test breaks in the manufacturing process and fiber splice problems in the installation process. More importantly, even very small airlines with diameters of less than 1 micron in the core region of the fiber can affect the transmission characteristics of the optical fiber such as loss, polarization mode dispersion (PMD), and Optical Time Domain Reflectometry (OTDR).
Techniques for detecting defects in fibers and, incidentally, dealing with the effects of defects on fiber diameter measurements, are known. See, for example, U.S. Pat. No. 4,046,536, issued Sep. 6, 1977, to D. H. Smithgall (analysis of fringe counts in the presence of xe2x80x9cdropoutsxe2x80x9d resulting from faults in the fiber); U.S. Pat. No. 4,501,492, issued Feb. 26, 1985, to N. Douklias (detection of fiber defects and testing of fiber diameters using a spatial filter prepared using diffracted/scattered light from a defect-free fiber); U.S. Pat. No. 5,185,636, issued Feb. 9, 1993, to L. J. Butten, et. al. (detection of fiber defects using light scattered from a fiber diameter measurement unit and performing Fast-Fourier-Transform (FFT) to examining the spectrum); and U.S. Pat. No. 5,880,825, issued Mar. 9, 1999, to Jakobsen et. al. (detection of fiber defects using light scattered from a fiber diameter measurement unit).
Although these techniques can detect defects in optical fibers, they nonetheless have several significant limitations, including cost and complexity. The added cost and complexity of such methods are due, in large part, to computational requirements and expenses associated with analyzing either the direct image or the frequency spectrum of light scattered signals, for example, performing a FFT on the light scattered signal repeatedly.
U.S. Pat. No. 6,313,909 discloses methods and apparatuses for detecting defects, such as air lines, in optical waveguide fibers. The methods and apparatus employ scattered light interference signals produced by a fiber clad measurement system that transversely illuminates a fiber with a laser beam. Defects in the fiber produce characteristic peaks in the frequency spectrum of the scattered light signal. By filtering the scattered light signal to separate the components associated with the fiber clad measurement system and the fundamental component associated with the fiber, the defect-related components in the scattered light signal which represent defects in the associated fiber are determined. Once the presence of these defect-related components is determined, a defect detection output pulse is generated for each such event. Although the ""909 patent uses the fiber""s frequency spectrum of a scattered light signal to identify defect-related components, the detector does not differentiate between airlines in a fiber core region from airlines in the overclad region. Additionally, the ""909 patent fails to disclose or teach how to dynamically adjust thresholds for the defect detection. Moreover, the system of the ""909 patent utilizes a low frequency signal as reference signal, which is not a robust indication of signal strength variations due to fiber vibration, different fiber draw towers, and different clad diameter measurement systems.
With increased market competition and heightened customer expectations, it has become important to develop a low-cost method to detect defects in optical fiber as it is drawn in the manufacturing process and deal with such defects in the fiber accordingly.
The presence of an airline or defect increases the signal strength within scattered light interference signals produced by a fiber clad measurement system that transversely illuminates a fiber with a laser beam, and more particularly, within the frequency bandwidth where the airline peak appears. However, because the overall scattered signal strength also fluctuates as the fiber vibrates during the draw process, it is difficult to determine whether the increase of signal strength alone is due to airline or fiber movement. The present invention solves this problem by using the signal around the clad diameter peak as a reference signal because an airline has little impact on the clad diameter signal while the vibration of the fiber affects both the clad signal and the airline signal. As a result, the normalized regular airline signal, which is defined as the ratio between the regular airline signal strength (from airlines outside of the core or in the overclad region) and the clad diameter signal strength, and the normalized core airline signal, which is defined as the ratio between the core airline signal strength and the clad diameter signal strength, remain the same even though the fiber vibrates. Thus, the increase of the normalized regular airline signal and the normalized core airline signal is a clear indication of the presence of a regular airline and a core airline, respectively.